Certain living species release and detect natural chemicals which act as signaling methods to other like neighbors. Cell to cell communication (quorum-sensing) within bacterial populations can direct certain internal processes, such as cell division, sporulation, genetic transformation and virulence. Similar signaling molecules, such as plant hormones, also control the way plants grow and develop. Bacteria, plant species and other micro-organisms release small signaling molecules into their intercellular space to communicate both with their interspecific and intraspecific neighbors. Certain bacteria and micro-organisms grouped in critical populations exhibit more information via the signaling molecules than the individual bacterium or micro-organism.
Traditional testing methods for bacteria are relatively expensive and time consuming. Most common testing methods require an environmental or product sample which is incubated in a separate media until enough bacteria exist to visually confirm their presence via culture plates or more elaborate immunoassays. These known methods are not real-time bacteria detection schemes. Other known detection methods, such as polymerase chain reaction (PCR), are faster but require a more complicated and expensive procedure. Many known bioassay sensors are not robust enough and therefore not suited for portable applications, because they require specific growing media to operate. Known biosensors also have difficulty adequately stating a limit of detection or dynamic range. The time to prediction of bacteria also depends on the response time of the bacteria cell growth.
Bacteria are single celled organisms typically 0.5 to 1 micron (μm) in diameter to 3-15 μm long (C. A. Hart, “Microterrors” Firefly Books Ltd, 2004) and are less mobile in their intercellular space than their small signaling molecules categorized as autoinducers. Acoustic wave devices have been developed for the direct detection of large bacterium, as described by Sang-Hun Lee, Desmond D. Stubbs, John Cairney, and William D. Hunt in “Rapid Detection of Bacterial Spores Using a Quartz Crystal Microbalance (QCM) Immunoassay” IEEE SENSORS JOURNAL, VOL. 5, NO. 4, AUGUST 2005. Lee et al., describe a method of instant identification of Bacillus subtilis (nonpathogenic simulant for Bacillus anthracis) bacterium by constructing a dual quartz crystal microbalance (QCM) immunosensing system. A set of 10-MHz AT-cut QCMs operating in thickness shear mode are employed in an enclosed flowcell. However, this method only detects the presence of a micro-organism, not the purpose of the micro-organism, such as cell division, sporulation, genetic transformation, virulence and species development.